1. Field of the Invention
This disclosure relates to position tracking systems that use wireless signals to determine the location of user equipment, such as GPS and A-GPS systems.
2. Related Art
Mobile communication devices (generally termed “user equipment” or “UE”) have become an everyday part of modern life. Cell phones, particularly, have become ubiquitous, to such an extent that some people are foregoing conventional telephone service in favor of their cell phones.
In addition to their communication capabilities, some cell phones now have the capability of determining the position of the user equipment, taking advantage of satellite positioning systems (SPS) and/or the known location of cellular base stations. Specifically, cell phones may be required to have some positioning capabilities in order to facilitate emergency response to 911 calls; in addition, some cell phones can respond to a user request for determining position. Whatever the reason, the increasing importance of position location services has encouraged development of rapid, high sensitivity methods for acquiring the signals used to determine position.
Position location technologies typically utilize wireless positioning signals concurrently transmitted from known locations. In GPS systems, the positioning signals are concurrently transmitted from a multiplicity of satellites at a known time, with a predefined frequency. On the ground, a GPS receiver acquires a positioning signal from each satellite within its view of the sky. The times of arrival of the positioning signals along with the exact location of the in-view satellites and the exact times the signals were transmitted from each satellite are used to triangulate the position of the GPS receiver.
When a SPS fix is made, it gives the position at the time the GPS signals are received. If, however, the GPS receiver is moving, it may be desirable to regularly update position. For example, if a user has requested directions to a location, it may be desirable to monitor the position of the UE in order to accurately pinpoint its approximate current location, and thereby more accurately direct the user to the intended destination. In such a circumstance, it is desirable to monitor position of the UE in real time as much as possible.
In order to monitor position of the UE over a period of time, the UE can make a series of GPS position fixes. Typically these position fixes are performed at a fixed, predetermined update rate, which may for example be at the maximum possible rate (e.g., immediately after a position fix is completed, the next position fix begins). Unfortunately, updating at the maximum possible rate can consume system resources and slow operation of the UE. Furthermore, reducing power consumption is an important issue for those portable devices which carry a battery, and updating the GPS position at the maximum possible update rate can draw significant power, which will reduce time before recharging will be required. For example, if a battery can store 600 mAHr, and a continuously-updating position location system consumes 45 mA per hour, then the battery would be completely consumed in about thirteen hours by the position location system alone. However, if the fixed update rate were to be reduced substantially to save power, then the update rate may not be sufficient to accurately track position, especially if the user equipment is moving quickly.
Generally, because power consumption is an important issue for portable devices such as mobile phones, any reduction in power consumption can advantageously reduce drain on battery power, thereby extending battery life, allowing battery size to be reduced, or both. For a user, an extended battery life allows more calls and position locations to be made before recharging is required. If battery size is reduced, a portable device can be made smaller, more lightweight, and can be manufactured at a lower cost.